One of the more unique and sought after early National receivers is the National SW-5, labeled by National in their marketing as the “Thrill Box”. The National SW-5 regenerative receiver was first announced to the world in the June 1930 issue of Radio News written by James Millen and Robert S. Kruse. Millen also authored an excellent description of the SW-5 and its underlying principles for the 1930 "Manual of Shortwave Radio". This article can be downloaded and viewed in Adobe PDF format by clicking on http://www.brucehowes.com/swr1930.pdf See the menu buttons on the left for more information on the SW-5.
The SW-5 is not an exceptionally attractive radio like the later HRO series. Rather than sporting the HRO and SW-3 black crackle paint, the National SW-5 lives inside of a rather plain rectangular metal cabinet finished in a dull textured brown paint color. Indeed the SW-5 bares a closer resemblance to metal breadbox than to a radio receiver. I’m not certain why National chose this color, but I suspect that the colors’ lack of visual appeal may have something to do with the scare number of SW-5s around today, for the performance of the receiver is exceptional but the aesthetics are quite plain, other than the National “diamond” painted on the top and sides of the cabinet.
Background and Design My receiver was first tested and put into service on July 6, 1931 by Jackson Research Labs laboratory engineer Calvin Foss. It appears that each SW-5 was tested as it came off the line and the results noted on a small chart on the right upper side of the top inside cover. The first stations logged on my unit were "PCP" in Holland and "WSE" in Argentina at 12:45PM July 6, 1931. The use of "Jackson Research Laboratories" on the testing tag struck me as curious, but upon further research I understand that National "subcontracted" the building of some of their earlier receivers to Jackson. It would seem that Jackson Laboratories was really just a “shell company”, located around the corner from National, on Jackson Street in Malden, MA. The reason for this scheme was apparently to avoid paying RCA patent royalties! Under the RCA agreement royalties had to be paid based on gross sales, regardless if all units sold used the patent or not. This was a clever tactic on RCA’s part, matched by an equally clever National response. Sadly it would seem that the SW-5 enjoyed only a short life, production given variously as ending in 1932 or 1934.
In the September 1930 issue of QST Leeds Radio offered the SW-5 as a battery kit for $75.00, an A.C. kit for $79.50 (wired $10.00 extra) and the power supply (less tube) for the A.C. model for $34.50. Aside from the kit form, there are at least three variants of the SW-5 design. The first was the battery set, then an A.C. set using a pair of 27 vacuum tubes in the audio output, and finally another A.C. set with a pair of 45 tubes for the audio. The rest of the tube compliment is the same between the set variations, consisting of an R.F. amplifier, a regen detector, and an audio voltage amplifier. A similar set, the National SW-58 was later offered, but the front panel tuning dial had changed from the early vertical style, know by National as type “H” as shown on my unit, to a slide rule layout, quite similar to the National FB-7 series.
My receiver seems to be the model marketed to the amateur radio community as it uses a pair of 27s in push-pull arrangement for audio. It came with five sets of plug-in coils covering the upper end of the commercial broadcast band to 25 meters. The general coverage coils, identical in construction, are used on the detector and oscillator stages, just like the later National SW-3 series. The bandspread coils utilized a small trimmer cap inside one of the coils. During the past 75 years, this unit had somehow managed stay quite intact and even included the original external “dog house” shaped power supply.
Millen and the early set designers at National felt that an outboard power supply was the only acceptable method to keep the receiver free of hum and drifting from heat. These concerns, hum and drifting from component heating, are why the very early National gear, SW-3, FB-7s, 1-10 and HRO all require an external power supply. In Millen’s 1930 Radio News article, he listed the design criteria of the SW-5 to be “1- Absolutely hum-less a.c. operation, 2- Single dial control, 3- Loud speaker reception from broadcast stations, 4- Good tone quality, 5- Non-critical tuning, 6- Neat appearance.” Based on my use of the SW-5, Millen seems to have hit it right on the mark as the set works remarkably well for a radio over 75 years old!
Use of the SW-5 The SW-5 is a fun receiver to use in today’s vintage hamshack. The unit is simple to tune, forgiving, and has plenty of audio output. A standard ¼ inch headphone jack is located on the rear panel but the unit really shines when played through a vintage speaker. Perhaps the only “quirky” item on the set is the antenna trimmer capacitor. Adjustment of this can cause the receiver to “pull” and skew the tune of the logging scale, so it is best to peak the trimmer on the signals of the band in use and leave it alone. One short coming, when using the SW-5 with a transmitter, is the lack of provision for muting. No external connection is provided, though it would be a simple matter to implement such a scheme by utilizing the headphone jack hole, or by snaking a cable out under the chassis.
The SW-5, like most regens, is a very “hot” receiver, capable of exceptional sensitivity. It is however, again like most regens, not capable of great selectivity. In fact, on the crowded AM broadcast band, it is very easy to tune in two stations at the same time. Still, given this shortcoming, the receiver is really quite simple to operate and capable of some surprisingly good audio. As the dial is simply calibrated as a logging scale, it helps to make up a “cheat sheet” of the tuning settings for quick return to desired frequencies. On this receivers maiden voyage in the W1UJR shack, I quickly developed just such a sheet and found that I was able to return to each frequency with good repeatability, a must for a serious monitoring receiver.
Once you find a SW-5, the next challenge is to locate the coil sets. While you can still find coil forms from time to time at hamfests, the higher bands, like 160 and the AM broadcast bands, require some rather complex winding to make the coil sets. So by all means, if you do come across a SW-5 or its little cousin the SW-3, try to get the coil sets at the time of acquisition. National offered surprising coverage with the SW-5 and SW-3 sets, with coils spanning from 90 KC to 35 MC. The bandspread coils, noted by the trimmer capacitor installed in the top of one of the coils, are very helpful if the set is be used on the ham bands.
The photos are of my set in the "as found" condition, prior to any cleaning or prep work, actually very little was needed. Interestingly enough the unit played out right of the box, pulling in domestic AM broadcast stations on the upper end of the band using only a 12 inch length of wire on the test bench. Perhaps it’s the novelty of the unit, or its ungainly appearance, but the SW-5 has become one of my favorite receivers of the National line.
I’ll write more about the SW-3 in an upcoming column, but for now, with hamfest season on the horizon, keep your eyes peeled for that ugly brown box hiding under someone’s table, you might just be in for a treat!
National SW-5 "Thrill Box"
